1. Field of the Invention
The present invention relates to a bonding-type surface-mount crystal oscillator that has a quartz crystal unit and a mounting substrate bonded to the bottom surface of the crystal unit, the mounting substrate housing an integrated circuit (IC) chip. In particular, it relates to a surface-mount crystal oscillator that prevents occurrence of horizontal misalignment between the mounting substrate and the crystal unit.
2. Description of the Related Arts
Surface-mount crystal oscillators, which integrally incorporate a quartz crystal unit and an IC chip having an oscillating circuit that uses the crystal unit, are small and light-weight and thus widely used as a frequency or time reference for a portable electronic apparatus including cellular phones. One example of the surface-mount crystal oscillators is the bonding type, which comprises: a crystal unit having a crystal blank hermetically sealed in a container; and a mounting substrate housing an IC chip and bonded to the bottom surface of the crystal unit. Typically, the mounting substrate is bonded to the crystal unit by soldering.
FIG. 1A is a cross-sectional view of a conventional bonding-type surface-mount crystal oscillator. FIG. 1B is a bottom view of a crystal unit used in the crystal oscillator. FIG. 1C is a top view of a mounting substrate.
The crystal oscillator shown has quartz crystal unit 1 and mounting substrate 2, which have the same horizontal outer dimensions. Crystal unit 1 has container body 3 having a recess and having the shape of a substantially rectangular parallelepiped. Crystal blank 4 is housed in the recess, and the recess is closed by metal cover 5, thereby hermetically sealing crystal blank 4 in container body 3. Container body 3 is made of laminated ceramics. On the inner bottom surface of the recess of container body 3, a pair of crystal holding terminals (not shown) are provided. At each of the four corners of the outer bottom surface of container body 3, external terminal 6 used for bonding mounting substrate 2 to crystal unit 1 is provided.
Crystal blank 4 is a substantially rectangular AT-cut quartz crystal blank, for example. An excitation electrode (not shown) is provided on each of the opposite principal surfaces of crystal blank 4, and leading electrodes extend from the respective excitation electrodes to the opposite sides of one end of crystal blank 4. The opposite sides of one end of crystal blank 4, to which the leading electrodes extend from the excitation electrodes, are secured by conductive adhesive 12 to the crystal holding terminals on the inner bottom surface of the recess of container body 3, and thus crystal blank 4 is held in the horizontal position in the recess as shown. The crystal holding terminals are electrically connected to two of four external terminals 6 on the outer bottom surface of container body 3 that are located at a pair of diagonal corners of the outer bottom surface via a conductive path formed on container body 3. The conductive path is formed on the lamination plane of the laminated ceramics and the outer side surface of container body 3.
The edges between every two adjacent surfaces of the four outer side surfaces of container body 3 are formed as arch-shaped chamfered portion 7 by through-hole processing. End-face electrodes 6a, on which a solder fillet is formed when mounting substrate 2 is bonded to crystal unit 1 by soldering, are formed on chamfered portions 7, that is, the through-hole-processed surfaces.
Metal cover 5 is bonded to the surface around the opening of the recess of container body 3 by seam welding or the like. Of external terminals 6 of container body 3, external terminals that are not connected to the crystal holding terminals are electrically connected to metal cover 5 via a through hole formed in container body 3 or the like.
In crystal unit 1, external terminals 6, the crystal holding terminals and end-face electrodes 6a are formed as electrode layers having a thickness about 5 μm and made of a base layer of tungsten (W) or the like plated with nickel (Ni) or gold (Au).
Mounting substrate 2 is made of laminated ceramics and has a substantially rectangular planar shape. Mounting substrate 2 has recess 2a for housing IC chip 8 in one principal surface, or specifically the top surface, thereof. At the four corners of the outer bottom surface, that is, the other principal surface, of mounting substrate 2, there are provided mounting terminals 9 that are used when the crystal oscillator is surface-mounted on a wiring board of a cellular phone or the like. Mounting terminals 9 include a power supply terminal, an output terminal and a ground terminal. At the four corners of the one principal surface of mounting substrate 2, that is, the four corners of the surface surrounding the opening of recess 2a, bonding terminals 10 are provided to correspond to external terminals 6 on crystal unit 1. Mounting terminals 9 and bonding terminals 10 are electrically connected to circuit terminals (not shown) formed on the inner bottom surface of recess 2a via a conductive path formed on mounting substrate 2.
IC chip 8 has a substantially rectangular shape and has at least an oscillating circuit that uses crystal unit 1 and that is integrated with a semiconductor substrate. Of the opposite principal surfaces of IC chip 8, the surface on which an electronic circuit, such as the oscillating circuit on the semiconductor substrate, is formed is refereed to as a circuit-forming surface. On the circuit-forming surface, a plurality of IC terminals for connecting IC chip 8 to an external circuit are formed. IC chip 8 is bonded to the bottom surface of recess 2a of mounting substrate 2 with the circuit-forming surface facing the bottom surface of recess 2a by bonding the IC terminals to the circuit terminals on the bottom surface of recess 2a by flip-chip bonding, such as ultrasonic thermocompression bonding. As a result, the electronic circuit in IC chip 8 is electrically connected to mounting terminals 9 and bonding terminals 10 via the circuit terminals. In mounting substrate 2, the circuit terminals, mounting terminals 9 and bonding terminals 10 are formed as electrode layers having a thickness about 5 μm and made of a base layer of tungsten (W) or the like plated with nickel (Ni) and gold (Au).
Cream solder 11 is applied to bonding terminals 10 on the top surface of mounting substrate 2 and external terminals 6 on the bottom surface of crystal unit 1, bonding terminals 10 and external terminals 6 are brought into intimate contact with each other, and reflow soldering is conducted by heating mounting substrate 2 and crystal unit 1 in a high-temperature furnace and then cooling the same, thereby bonding mounting substrate 2 housing the IC chip and crystal unit 1 to each other, thereby completing the crystal oscillator. The completed crystal oscillator is then mounted on a wiring board by reflow soldering.
However, the above-described crystal oscillator in which crystal unit 1 and mounting substrate 2 are bonded by soldering has a problem that, when the crystal oscillator is mounted on a wiring board by reflow soldering, solder 11 bonding crystal unit 1 and mounting substrate 2 can also be molten by the heat, thereby causing misalignment of crystal unit 1 with mounting substrate 2 particularly in the horizontal direction.
As a solution to the problem, Japanese Patent Laid-Open Nos. 10-98151 and 2001-7647 (JP-A-10-098151 and JP-A-2001-007647) disclose an arrangement in which the mounting substrate and the crystal unit are bonded to each other by an insulating adhesive in addition to being electrically bonded to each other by soldering. However, this arrangement requires a step of applying the insulating adhesive and the like, and therefore, the manufacturing process becomes complicated.